[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |akobets at mail dot ru

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> 2013-01-31 21:46:42 UTC ---
*** Bug 56165 has been marked as a duplicate of this bug. ***

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> 2013-02-03 02:18:59 UTC ---
*** Bug 56165 has been marked as a duplicate of this bug. ***

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[luto at mit dot edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

Andy Lutomirski <luto at mit dot edu> changed:

           What    |Removed                     |Added
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                 CC|                            |luto at mit dot edu

--- Comment #9 from Andy Lutomirski <luto at mit dot edu> ---
In this case, perhaps sibling call optimization is the wrong thing here.  The
caller of a noreturn function shouldn't pop the stack, but it also shouldn't
save registers (and, if it doesn't need to save registers, it shouldn't create
a stack frame in the first place).

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jay.foad at gmail dot com

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 58152 has been marked as a duplicate of this bug. ***

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |hjl.tools at gmail dot com

--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 67327 has been marked as a duplicate of this bug. ***

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

--- Comment #12 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Richard Henderson from comment #2)
> Working as designed.
> (1) It often takes more insns to pop the stack frame than to make the call.
> (2) You get a proper backtrace from abort.
> (3) http://gcc.gnu.org/ml/gcc-patches/2000-10/msg00180.html

Glibc has been using

ENTRY (__memmove_chk)
        movl    12(%esp), %eax
        cmpl    %eax, 16(%esp)
        jb      __chk_fail
        jmp     memmove
END (__memmove_chk)

since 2004. #1 and #2 shouldn't be the reason not to optimize.  I
am using:

/* Due to
   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837
   noreturn attribute disable tail call optimization.  Removes noreturn
   attribute to enable tail call optimization.  */
extern void *chk_fail (void) __asm__ ("__chk_fail") attribute_hidden;

to work around this.

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |lukas.graetz@tu-darmstadt.d
                   |                            |e

--- Comment #14 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
*** Bug 111786 has been marked as a duplicate of this bug. ***

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[goon.pri.low at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

gooncreeper <goon.pri.low at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |goon.pri.low at gmail dot com

--- Comment #15 from gooncreeper <goon.pri.low at gmail dot com> ---
May I suggest we just add something like __attribute__((trace)) for the special
abort case? Noreturn was added for code optimization after all, not for
backtracing.

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=83324
                 CC|                            |xry111 at gcc dot gnu.org

--- Comment #16 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to gooncreeper from comment #15)
> May I suggest we just add something like __attribute__((trace)) for the
> special abort case? Noreturn was added for code optimization after all, not
> for backtracing.

It will break any attempts to debug an abort until the libc headers are updated
to use __attribute__((trace)).

Note that in GCC noreturn has been added far before the WG14 _Noreturn paper
(even this ticket predates the WG14 paper), so the rationale in the paper may
not apply.

In practice most _Noreturn functions are abort, exit, ..., i.e. they are only
executed one time so optimizing against a cold path does not help much.  I
don't think it's a good idea to encourage people to construct some fancy code
by a recursive _Noreturn function (why not just use a loop?!)  And if you must
write such fancy code anyway IMO musttail attribute (PR83324) will be a better
solution.

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[lukas.graetz@tu-darmstadt.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

--- Comment #17 from Lukas Grätz <lukas.graetz@tu-darmstadt.de> ---
(In reply to Xi Ruoyao from comment #16)
> (In reply to gooncreeper from comment #15)
> > May I suggest we just add something like __attribute__((trace)) for the
> > special abort case? Noreturn was added for code optimization after all, not
> > for backtracing.
> 
> It will break any attempts to debug an abort until the libc headers are
> updated to use __attribute__((trace)).

"any attempts"? We could simply use the gdb debugger and ignore the backtrace.
In comparison, the backtrace is a rather restricted debugging instrument.

If there are applications that really depend on GCC's backtrace, this should be
the reason to keep the current behaviour.

> 
> Note that in GCC noreturn has been added far before the WG14 _Noreturn paper
> (even this ticket predates the WG14 paper), so the rationale in the paper
> may not apply.

Backtracing functionality is highly platform dependent, so there is no surprise
that the C standard cannot guarantee anything about it.

> 
> In practice most _Noreturn functions are abort, exit, ..., i.e. they are
> only executed one time so optimizing against a cold path does not help much.
> I don't think it's a good idea to encourage people to construct some fancy
> code by a recursive _Noreturn function (why not just use a loop?!)

... and why not just if and goto? Because it is considered good programming
practice to structure source code into functions (not to long) and loops. If a
function gets too big, GCC might not optimize it well.

>  And if
> you must write such fancy code anyway IMO musttail attribute (PR83324) will
> be a better solution.

I agree.

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[lukas.graetz@tu-darmstadt.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

--- Comment #18 from Lukas Grätz <lukas.graetz@tu-darmstadt.de> ---
On another thought: I think something like -fignore-backtrace could be a
reasonable optimization flag (enabled by default for -O4). By ignoring the
backtrace we could do other optimizations on size and speed, like in this
ticket and duplicates.

There are use cases for that, see some of the duplicate tickets. For example in
PR56165, they didn't want to support any debugging at all. And even if you want
debugging, you might want to disregard backtraces and use a more sophisticated
debugging device. This is independent from attribute musttail, with
-fignore-backtrace we would leave GCC more freedom to do optimization.

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[pskocik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

Petr Skocik <pskocik at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |pskocik at gmail dot com

--- Comment #19 from Petr Skocik <pskocik at gmail dot com> ---
IMO(In reply to Xi Ruoyao from comment #16)

> In practice most _Noreturn functions are abort, exit, ..., i.e. they are
> only executed one time so optimizing against a cold path does not help much.
> I don't think it's a good idea to encourage people to construct some fancy
> code by a recursive _Noreturn function (why not just use a loop?!)  And if
> you must write such fancy code anyway IMO musttail attribute (PR83324) will
> be a better solution.

There's also longjmp, which may not be all that super cold and may be executed
multiple times. And while yeah, nobody will notice a single call vs jmp time
save against a process spawn/exit, for a longjmp wrapper, it'll make it a few %
faster (as would utilizing _Noreturn attributes for better register allocation:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114097, which would also save a
bit of codesize too). Taillcalls can also save a bit of codesize if the target
is near.

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[lukas.graetz@tu-darmstadt.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

--- Comment #20 from Lukas Grätz <lukas.graetz@tu-darmstadt.de> ---
(In reply to Petr Skocik from comment #19)
> IMO(In reply to Xi Ruoyao from comment #16)
>  
> > In practice most _Noreturn functions are abort, exit, ..., i.e. they are
> > only executed one time so optimizing against a cold path does not help much.
> > I don't think it's a good idea to encourage people to construct some fancy
> > code by a recursive _Noreturn function (why not just use a loop?!)  And if
> > you must write such fancy code anyway IMO musttail attribute (PR83324) will
> > be a better solution.
> 
> There's also longjmp, which may not be all that super cold and may be
> executed multiple times. And while yeah, nobody will notice a single call vs
> jmp time save against a process spawn/exit, for a longjmp wrapper, it'll
> make it a few % faster (as would utilizing _Noreturn attributes for better
> register allocation: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114097,
> which would also save a bit of codesize too). Taillcalls can also save a bit
> of codesize if the target is near.


Just to emphasize, tail call optimization is not just for speed. It is
essential to avoid waste of stack space. Especially, to avoid potential stack
overflows, it should _not_ be necessary to replace all recursions with loops,
as Xi Ruoyao suggests. Ah, and I also think that recursions in C is not fancy
(anymore), since everyone expects the compiler to do sibcall or similar
optimizations. Noreturn functions are the exception for that. So it would be
consequent indeed to do sibcall optimization for noreturn functions, too!

Personally, I would be satisfied with the new attribute musttail to enforces
tail calls whenever necessary (given that this will be available for C, not C++
only). But speed-wise, musttail might not have the desired effect. It is meant
for preserving stack space.

---

Following Petr Skocik, I quick-tested on my computer:

===== longjmp_wrapper.c =====================
#include <setjmp.h>

__attribute__((noreturn))
void longjmp_wrapper(jmp_buf env, int val) {
    longjmp(env, val);
}

===== longjmp_main.c ========================
#include <setjmp.h>
#include <limits.h>

__attribute__((noreturn))
void longjmp_wrapper(jmp_buf env, int val);

int main(void) {
    jmp_buf env;
    for (int i = 0; i < INT_MAX; i++) {
        if (setjmp(env) == 0) {
            longjmp_wrapper(env, 1);
        }
    }
}
=============================================

After compiling with

$ gcc -O3 -m32 -c -S longjmp_wrapper.c -o longjmp_wrapper.S

I copied and manually modified the generated longjmp_wrapper.S as follows:

9,15c9
<       subl    $20, %esp
<       .cfi_def_cfa_offset 24
<       pushl   28(%esp)
<       .cfi_def_cfa_offset 28
<       pushl   28(%esp)
<       .cfi_def_cfa_offset 32
<       call    longjmp
---
> 	jmp 	longjmp


Then I compiled both versions with longjmp_main.c, again with -m32. Measured
with "time", the sibcall and unmodified version took around 23.5 sec and 24.5
sec on my computer. So around 4 % improvement for 32 bit x86. For 64 bit x86,
both took around 18 secs without noticeable speed difference (perhaps because
both arguments are passed in registers instead of stack by 64 bit calling
conventions).

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[pinskia at gcc dot gnu dot org]

------- Comment #6 from pinskia at gcc dot gnu dot org  2007-12-26 09:36 -------
*** Bug 34589 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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                 CC|                            |vyzo at media dot mit dot
                   |                            |edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[kauer at os dot inf dot tu-dresden dot de]

------- Comment #5 from kauer at os dot inf dot tu-dresden dot de  2007-08-28 19:27 -------
(4) abort produce a proper backtrace, if it is not a noreturn function
(5) this optimization could be configurable by something like
-foptimize-noreturns


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[kauer at os dot inf dot tu-dresden dot de]

------- Comment #4 from kauer at os dot inf dot tu-dresden dot de  2007-08-28 19:07 -------
Some comments from the outside:
(1) It should be possible to exchange the emmited call into a jmp, without
adding the function epilog.
(2) A proper backtrace is not generated on optimized sibling functions.
(3) http://gcc.gnu.org/ml/gcc-patches/2000-10/msg00180.html explains, that it
is hard to implement


-- 

kauer at os dot inf dot tu-dresden dot de changed:

           What    |Removed                     |Added
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                 CC|                            |kauer at os dot inf dot tu-
                   |                            |dresden dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837

[Bug rtl-optimization/10837] noreturn attribute causes no sibling calling optimization

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2007-08-16 10:38 -------
*** Bug 33083 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |us15 at os dot inf dot tu-
                   |                            |dresden dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837